In the manufacture of printed circuit boards, various substrate protecting means, such as resist used in the etching step and solder resist used in the soldering step, have conventionally been required. Also in the manufacturing process of a film-like printed circuit board (flexible printed circuit board abbreviated as FPC) used in small equipment, a solder resist for protecting independent wirings is required in the soldering step for mounting parts.
As a protective means for these substrates, there has conventionally been used a cover-layer for laminating those obtained by punching out a polyimide film into a predetermined shape, or a cover-coat made of a heat-resistant material for applying ink. This cover-layer or cover-coat also serves as a protective film for wirings after soldering and requires heat resistance during soldering, insulating properties, and sufficient pliability so as not to crack when folded during incorporation of a substrate. An FPC used for purposes other than battery driving equipment also requires flame resistance, and those having good balance with the pliability are required.
The cover-layer formed by punching out the polyimide film meets the above prescribed properties and is used most popularly at present, but has a problem in that the punching out step requires an expensive mold and manual positioning and lamination of the punched out film increase the cost, and moreover, a micropattern is difficult to form. The cover-coat has such a problem in that the manufacturing cost increases because screen printing requires a drying step and the operatability becomes worse.
To solve these problems, there has been proposed a method of applying a photosensitive resin composition on a substrate in the form of a liquid or laminating it in the form of a film. According to this method, since a cover-coat or cover-layer of a micropattern can be easily formed by forming a coating on the substrate and subjecting to the exposure, development, and heating steps using a photographical technique, various photosensitive resin compositions have conventionally been developed.
However, no conventional photosensitive resin composition met all the properties required for an FPC. For example, a photosensitive resin composition comprising a prepolymer obtained by the addition reaction of a novolak type epoxy vinyl ester resin and a polybasic anhydride, a photopolymerization initiator, a diluent and an epoxy compound has been proposed (Japanese Exampled Patent Application, Second Publication No. 1-54390). This photosensitive resin composition had good heat resistance and insulating properties, but is not suited for use in an FPC because of poor pliability. Also, there has been proposed a photosensitive resin composition prepared by mixing a binder system composed of a low-molecular weight copolymer (which is a reaction product of a copolymer formed from an ethylenically unsaturated dicarboxylic anhydride and an ethylenically unsaturated comonomer, and an amine) and a carboxylic acid-containing high-molecular weight copolymer with an acrylated urethane monomer component, a photoinitiator and a block polyisocyanate crosslinking agent (Japanese Unexamined Patent Application, First Publication No. 7-278492). However, this photosensitive resin composition had a problem that the use is limited because of its poor flame resistance.
As a method of imparting the flame resistance to the photosensitive resin composition, there have conventionally been used a method of using a halide-based flame retardant such as brominated epoxy resin and a method of using a flame retardant of a combination of the halide-based flame retardant and an auxiliary flame retardant such as antimony trioxide (Japanese Unexamined Patent Application, First Publication No. 9-325490, and Japanese Unexamined Patent Application, First Publication No. 11-242331). However, these flame retardants are inferior in reliability in a high temperature environment and, when using an antimony compound, it is necessary to take into account environmental problems with respect to waste disposal of the resin. Furthermore, the brominated epoxy resin had a problem in that the pliability is impaired when incorporating it in a sufficient amount to obtain a satisfactory flame-retarding effect.
Also, methods of using a phosphate ester as the flame retardant have been proposed (Japanese Unexamined Patent Application, First Publication No. 9-235449, Japanese Unexamined Patent Application, First Publication No. 10-306201, and Japanese Unexamined Patent Application, First Publication No. 11-271967). However, a poor flame-retarding effect is exerted by using only the phosphate ester, and it is impossible to sufficiently meet criteria of the flame resistance according to the UL Standard.
As described above, it is not easy to obtain a resist film, which has both high flame resistance and pliability capable of meeting criteria according to the UL Standard and is also superior in soldering heat resistance, moisture resistance, high temperature reliability, photosensitivity, and developability. Therefore, a further improvement has been required.